Subset selection of RFID tags using light

ABSTRACT

Methods and apparatuses for selecting a subset of RFID tags are provided in some embodiments. These methods and apparatuses utilize the susceptibility to light by persistent nodes found in passive tags. Light can be used to intentionally reduce persistence times in a particular subset tags or even an individual tag. Then, persistent nodes can be used as a selection criterion to distinguish previously illuminated tags from non-illuminated tags. In other embodiments, a power circuit receives a RF input source and generates a direct current (DC) output voltage. The circuit includes a bias circuit to supply a gate to source bias, which is independent of the DC output voltage. The circuit further includes a voltage multiplier circuit that is coupled to the bias circuit. The voltage multiplier circuit has MOS transistors with one transistor to receive the gate to source bias.

This application is a continuation of co-pending U.S. patent applicationSer. No. 12/420,009 filed on Apr. 7, 2009, and is related to U.S.Provisional Patent Application No. 61/123,410, which was filed on Apr.7, 2008; this application claims the benefit of the provisional's filingdate under 35 U.S.C. §119(e) and is hereby incorporated herein byreference in its entirety.

GOVERNMENT RIGHTS NOTICE

Embodiments of the present invention were made with U.S. Governmentsupport under North Dakota State University Subcontract SPP002-5,Defense Microelectronics Activity (DMEA) Contract No. H94003-06-1-0601(prime). Certain embodiments, including light insensitive circuitstructures and transistor based voltage multipliers and demodulatorsdescribed herein, were made with U.S. Government support under DefenseMicroelectronics Activity (DMEA) Contract No. H94003-06-1-0612. The U.S.Government has certain rights to this invention.

FIELD OF THE TECHNOLOGY

The present invention generally relates to the field of radio frequencyidentification (“RFID”) devices, and specifically to techniques forsubset selection of tags for inventory or testing.

BACKGROUND

RFID labels, inlays, straps, and transponders (commonly referred toherein as “tags”) are widely used to associate an object with anidentification code. Tags generally combine one or more antennas with ananalog and/or digital electronic circuit chip (RFID chip) that mayinclude, for example, communications electronics, data memory andcontrol logic. Examples of RFID tag applications are automobilesecurity-locks, access control to buildings, inventorying and parceltracking. In general, RFID tags can retain and transmit enoughinformation to uniquely identify individuals, packages, inventory andthe like.

There are three basic types of RFID tags. A passive tag is a beampowered device which rectifies energy required for operation from radiowaves generated by a reader. For communication, the passive tag createsa change in reflectivity of the field which is reflected to and read bythe reader. This is commonly referred to as continuous wavebackscattering. A battery-powered semi-passive tag also receives andreflects radio waves from the reader; however a battery powers the tagindependent of receiving power from the reader. An active tag, having anindependent power supply, includes its own radio frequency source fortransmission.

Passive backscatter tags use voltage multipliers to convert RF signalsinto DC power to power the chips circuitry. The range of a passive RFIDtag is limited by its ability to convert low amplitude RF signals intosufficient DC power to power the tag's circuits.

RFID tags can also include sensors, such as vibration sensors,temperature sensors, and light sensors. As an example, a temperaturelogging RFID tag would periodically sample the temperature of itsenvironment and save the measured temperature to its memory. The readercan later read out this record of temperatures as well as otherinformation from the tag, such as its ID.

The reader, sometimes referred to as an interrogator, includes atransmitter to transmit RF signals to the tag and a receiver to receivetag modulated information. The transmitter and receiver can be combinedas a transceiver. Communications between a reader and tag is defined byan air interface protocol, such as (without limitation):

(i) EPCglobal's EPC Radio-Frequency Identity Protocols Class-1Generation-2 UHF RFID Protocol for Communications at 860 MHz-960 MHz,version 1.1.0(http://www.epcglobalinc.org/standards/uhfc1g2/uhfc1g2_(—)1_(—)1_(—)0-standard-20071017.pdf)(hereinafter referred to as the “UHF Gen2 standard”);

(ii) adaptations of the UHF Gen2 standard for operation at highfrequency (“HF”), for example at 13.56 MHz; and

(iii) ISO/IEC 18000-6 Information technology—Radio frequencyidentification for item management—Part 6: Parameters for air interfacecommunications at 860 MHz to 960 MHz, Amendment 1: Extension with Type Cand update of Types A and B. Each of the above protocols is incorporatedherein by reference for all purposes.

Communication protocols, such as these, may require that a passive tagoperate a timing circuit or maintain a flag value during a brief lapseof received power. For example, the UHF Gen2 standard requirespersistence for flags SL, S1, S2, and S3, but not S0. U.S. Pat. No.6,942,155, assigned to Alien Technology Corporation (“Alien,” also theassignee to this invention) and incorporated by reference herein for allpurposes, provides various teachings on persistent flags and nodes.Other or related techniques have been purportedly suggested by thefollowing patents (each of which is incorporated by reference herein forall purposes):

(i) U.S. Pat. No. 7,259,654; and

(ii) U.S. Pat. No. 7,215,251.

As discovered by Alien, persistent nodes suffer from a latentsusceptibility to light, even ambient light. That is to say, exposure tolight can dramatically decrease persistent time. During development ofthe inventions herein, the inventors recorded the results below fromconventional tags:

Measured UHF Gen2 Persistence Results (seconds) Persistent requirement40 W Sun IC Chip Node (seconds) Dark Ambient Light Light Vendor A S10.5-5.0 2.1 2.0 0.380 0.050 Chip S2 >2 87.0 9.0 0.070 <0.02 S3 >2 77.07.9 0.050 <0.02 SL >2 100.0 6.3 0.040 <0.02 Vendor B S1 0.5-5.0 2.7 2.00.130 <0.02 Chip S2 >2 41.3 30.0 0.990 0.370 S3 >2 33.8 30.0 1.900 0.510SL >2 38.8 20.0 0.800 0.320Opaque encapsulation of an integrated circuit chip can reduce thissusceptibility, but that may not be desirable or economical.

As an ostensibly unrelated problem, conventional readers generallyrequire complex anti-collision software in order to poll and identifytags when many tags are disposed in its RF field. However, even afteridentification of each tag, one is unable to respectively associateidentification information to each tag. In other words, conventionalreaders only indicate that a plurality of tags were read, not that aspecific tag contained specific data. An individual tag must bephysically or electrically isolated (e.g., shielded) to provide desiredgranularity in information. Readers can bear further complexity byimplementing range and bearing techniques as described in U.S. PatentApplication Publication No. 2005/0237953 (which is incorporated byreference herein for all purposes), assigned to Alien, to unambiguouslyassociate specific tags to received information.

SUMMARY OF THE DESCRIPTION

Methods and apparatuses for selecting a subset of RFID tags areprovided. These methods and apparatuses utilize the susceptibility tolight by persistent nodes found in passive tags. Light can be used tointentionally reduce persistence times in a particular subset of tags oreven an individual tag. Then, persistent nodes can be used as aselection criterion to distinguish previously illuminated tags fromnon-illuminated tags.

In one embodiment, a method for selecting a subset of RFID tagscomprises setting a persistent node for each of the plurality of passivetags to a first logic value. These persistent nodes are capable ofmaintaining, under dark lighting conditions, the first logic value inthe absence of power for a first time period. Next, power is interruptedto the passive tag for a second time period. The second time period isshorter than the first time period. A subset of the plurality of tags isilluminated with light during at least a portion of the second timeperiod. As a result, the persistent nodes of the subset change into asecond logic value before power is resumed. The subset can then bedistinguished by the second logic value for identification, testing orother processing.

In another embodiment of the invention an RFID system includes a tagwith an integrated circuit. Deep well implants and other techniques areused to make some parts of the integrated circuit insensitive to light.

In another embodiment of the invention an RFID system includes a tagwith an integrated circuit. Some parts of the integrated circuit areinsensitive to light, but made to vary with temperature. The temperaturevarying properties of the circuit are used by the reader to calculatethe temperature of the RFID tag.

In another embodiment of the present invention, an RFID system includesa tag with an integrated circuit. The integrated circuit maintainsduring brief absences of power at least one persistent flag. Thepersistence time of such flag is susceptible to light. The systemfurther includes an RFID reader with an appropriate light source toreduce the persistence time.

In yet another embodiment of the present invention, an RFID readerincludes a radio frequency source, an antenna to transmit radiofrequency waves generated by the radio frequency source, and a lightsource. The light source is configured to produce at least one lightbeam that alters or manipulates a function of an RFID tag. In a specificembodiment, the manipulated function involves a persistent node,persistent flag, or timing circuit.

In yet another embodiment of the present invention, a power circuitreceives a RF input source and generates a direct current (DC) outputvoltage. The circuit includes a first bias circuit to supply a gate tosource bias, which is independent of the DC output voltage. The circuitfurther includes a voltage multiplier circuit that is coupled to thefirst bias circuit. The voltage multiplier circuit has at least onen-channel metal-oxide-semiconductor (NMOS) transistor or at least onep-channel metal-oxide-semiconductor (PMOS) transistor. The voltagemultiplier circuit generates the DC output voltage for powering a RFidentification tag in accordance with one embodiment. The first biascircuit receives the RF input source and generates the gate to sourcebias for a gate terminal of one of the MOS transistors. A second biascircuit receives the RF input source and generates a gate to source biasfor a gate terminal of another one of the MOS transistors in the voltagemultiplier circuit.

Various additional objects, features, and advantages of the presentinvention can be more fully appreciated with reference to the detaileddescription and accompanying drawings that follow.

In yet another embodiment of the present invention, a circuit is madeinsensitive to light absorbed in the substrate of the chip by isolatingthe base of transistors in a grounded well. Electron hole pairs whichare generated by light in the great majority of the silicon material arethen captured by the junction of the well and recombine through thegrounded ohmic contacts. The generated photocurrents are thus divertedfrom sensitive high impedance nodes of the circuit.

In yet another embodiment of the present invention, electron hole pairswhich are generated by incident light are then captured by a junction.The photocurrent is then converted into a change of function of thecircuits of the RFID tag, and a reader uses information returned by thetag to infer the amount of light incident on the tag.

In yet another embodiment of the present invention, a device in thetag's integrated circuit changes in response to temperature. This changecauses a change of function of the circuits of the RFID tag, and areader uses information returned by the tag to infer the temperature ofthe tag.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and notlimitation in the Figures of the accompanying drawings in which likereferences indicate similar elements.

FIG. 1 illustrates a simplified block diagram of an RFID systemaccording to an embodiment of the present invention.

FIG. 2 illustrates a simplified example of an RFID system according toanother embodiment of the present invention.

FIG. 3 illustrates a simplified example of an RFID system according toyet another embodiment of the present invention.

FIG. 4 illustrates a simplified flowchart for subset selection accordingto an embodiment of the present invention.

FIG. 5 illustrates a simplified flowchart for subset selection accordingto another embodiment of the present invention.

FIG. 6 illustrates a schematic and cross sectional view of a circuitwith an analog storage device in accordance with one embodiment of thepresent invention.

FIG. 7 illustrates a cross sectional view of a circuit structure havingan n channel transistor in accordance with one embodiment of the presentinvention.

FIG. 8 illustrates a cross sectional view of a circuit structure havingan isolated p-well in accordance with one embodiment of the presentinvention.

FIG. 9 illustrates a schematic and cross sectional view of a p-channelcircuit for Analog Data Storage in accordance with one embodiment of thepresent invention.

FIG. 10 illustrates a cross sectional view of a circuit structure havinga PMOS transistor in accordance with one embodiment of the presentinvention.

FIG. 11 illustrates a circuit of a current biased oscillator inaccordance with one embodiment of the present invention.

FIG. 12 illustrates a conventional CMOS transistor doubler circuitimplementation in accordance with a prior approach.

FIGS. 13A and 13B illustrate CMOS transistor doubler circuitimplementations in accordance with certain embodiments of the presentinvention.

FIG. 14A illustrates a block diagram of a circuit 1400 that receives aRF input source and generates a DC output voltage in accordance withsome embodiments of the present invention.

FIG. 14B shows a schematic of a CMOS voltage multiplier circuit inaccordance with one embodiment of the present invention.

FIG. 15 shows a schematic of a doubler circuit suitable for use as ademodulator in accordance with one embodiment of the present invention.

FIG. 16 shows a schematic of a CMOS multi-stage voltage multipliercircuit in accordance with one embodiment of the present invention.

FIG. 17A shows a schematic of a CMOS voltage multiplier circuit inaccordance with another embodiment of the present invention.

FIG. 17B shows a schematic of a CMOS voltage multiplier circuit inaccordance with yet another embodiment of the present invention.

FIG. 18 illustrates an exemplary RFID system according to an embodimentof the present invention.

FIG. 19 shows an example of an RFID system according to one embodimentof the present invention.

FIG. 20 shows an exemplary embodiment of an RFID tag according to anembodiment of the present invention.

FIG. 21 shows an example of an RFID tag according to another embodimentof the present invention.

DETAILED DESCRIPTION

The following description and drawings are illustrative of the inventionand are not to be construed as limiting the invention. Numerous specificdetails are described to provide a thorough understanding of the presentinvention. However, in certain instances, well known or conventionaldetails are not described in order to avoid obscuring the description ofthe present invention. References to one or an embodiment in the presentinvention are not necessarily references to the same embodiment; and,such references mean at least one.

FIG. 1 illustrates a simplified block diagram of an RFID system 100according to an embodiment of the present invention. In this example,tag 102 is attached to container 103 (e.g., box, corrugated box, carton,or the like). Tag 102 is in close proximity to other tags affixed tosimilar containers, such as on a densely packed pallet. Due to thisproximity, handheld reader 104 using RF antenna 106 observes many tagsin its RF field.

In order to singulate tag 102, reader 104 illuminates tag 102 with lightbeam 110. Light beam 110 can alter the operation of tag 102, therebydistinguishing it from other tags in reader's field. In this embodiment,light beam 110 intentionally hastens the decay of at least onepersistent node from a first logic value to a second logic value. Reader104 can then selectively communicate with tag 102 using the second logicvalue as a selection criterion.

Light beam 110 can encompass any portion of the entire electromagneticspectrum, including ultraviolet (about 10 nm to 400 nm wavelength),visible (about 400 nm to 700 nm wavelength), infrared (about 700 nm and1 mm wavelength) radiation, or combination thereof. In a preferredembodiment, light beam 110 includes infrared radiation sinceconventional persistent nodes are particularly susceptible to suchradiation. The intensity of the light is at least 500 watt/m², or morethan at least 100 watt/m², or even more than at least 10 watt/m². Theintensity of light beam 110 influences the speed of decay in persistencetime. Increasing light intensity shortens persistence time—thiscorrelation need not be linear.

In FIG. 1, light source 108 is integrated with, or attached to, reader104. In alternative embodiments, light source 108 can be disposed awayfrom the reader and electrically coupled to the reader or commonlycontrolled by a host computer. Preferably, light source 108 includes amanual or automatic adjustable aperture to vary beam size. With a wideaperture, light beam 110 encompasses a subset of tags, while a narrowaperture singulates an individual tag. In a specific embodiment, lightsource 108 can be a light projector illuminating a plurality of tags ina pattern. The pattern can be (i) a predetermined pattern (e.g.,checkerboard, diagonal, crisscross, polygon, oval, rectangle, donutshaped with a lighted ring portion and an unlighted central portionetc.), (ii) a pseudo-randomly generated pattern, or (iii) a sequence orcombination of either (i) and/or (ii).

Light source 108 can use any type of source, including withoutlimitation:

(i) combustion light (e.g., argon flash, acetylene/carbide lamps,candles, fire, gas lighting, kerosene lamps, and lanterns);

(ii) direct chemical light (e.g., chemoluminescence (lightsticks),fluorescence, and phosphorescence);

(iii) electric light (arc lamps, incandescent lamps, flashlight, halogenlamps, electroluminescent lamps, light-emitting diodes (“LED”) (organiclight-emitting diodes, polymer light-emitting diodes, solid-statelighting), gas discharge lamps, fluorescent lamps, neon and argon lamps,plasma lamps, xenon flash lamps, and high-intensity discharge lamps); or

(iv) reflections of the above.

In fact, natural light (e.g., astronomical objects, solar radiation,skylight, sunlight, moonlight, or bioluminescence) can be used in lieuof, or in addition to, light source 108. In a preferred embodiment,light source 108 is at least one of an LED, fluorescent bulb, andincandescent bulb. In a more preferred embodiment, a spectrum of thelight source 108 includes wavelengths that will be absorbed by siliconto create electron hole pairs as described in more detail below with thelight insensitive circuit structures, such as an infrared LED.

System 100 provides many benefits in package handling, warehouse andretail environments. A user may need to determine the contents of aparticular package, and not others in its proximity. For example, inFIG. 1, the user may solely want to determine the contents of container103. She can easily do so under system 100 by illuminating tag 102.However, with conventional RFID systems, she would need to identify allthe containers within a reader's RF field and still not necessarily knowwhich specific container held a desired item.

For further utility, reader 104 can employ range and bearing techniquesin conjunction with light source 108 to visually indicate a desiredcontainer 103. Reader 104 can initially illuminate the locationindicated by range and bearing. As confirmation, reader 104 can nextverify that a persistent time associated with tag 102 has decreasedafter illumination. If a persistence time for tag 102 is not reduced,reader 104 can automatically re-direct light beam 110 to adjoining areasuntil tag 102 is confirmed to be illuminated by a decreased persistenttime. Alternatively, light source 108 can illuminate a large area andthen systematically decrease the area size until reader 104 determinesthat only tag 102 has reduced persistence, thus indicating that tag 102is the only tag within the light beam.

It should be clear from the teachings herein that a persistent flag is abit, character(s), or other indicator that signals the occurrence ofsome condition. The persistent node is a circuit which is initialized toa value, and the value read from the persistent node can change at somelater time. Persistent flags can be implemented using persistent nodesas described in one or more of the incorporated references. As anexample, persistent flags can be implemented essentially as a timerusing persistent nodes. For example in the ISO/IEC 18000-6cspecification, each flag has one of two values. “A” or “B” for the S1,S2 or S3 flags, and “asserted” or “deasserted” for the SL flag. They canoperate by charging a capacitor up to a certain level and thendischarging that capacitor over time. When the capacitor has a chargeabove a threshold level, it represents “B”; when the charge drops belowthat threshold level, it represents “A”. Thus, when the RF signal isturned off, if the capacitor charge is already below the thresholdsignifying “A”. any charge simply dissipates over time. Similarly, ifthe capacitor charge was above the threshold, signifying “B” when the RFsignal is turned off, the charge dissipates over time until thecapacitor is discharged or the RF power is turned back on. In both casesof this example, a transistor coupled to the capacitor controls the rateof discharge such that a “B” value would remain for the requiredpersistence time. In other words, the transistor acts as a clamp to cutoff the capacitor from the rest of the circuitry so as to allow a slowrate of discharge. Once the capacitor charge goes below the thresholdlevel required for a “B”, the value will become “A”.

FIG. 2 illustrates a simplified example of RFID system 200 according toanother embodiment of the present invention. Substrate 202 includes aplurality of inlays assemblies, including inlay assembly 204. In aspecific embodiment, inlay assembly 204 comprises a strap: a surfacemounted IC chip, strap contact pads electrically coupled to the IC chip,and the underlying portion of strap substrate. The surface mounted IC,or flip chip, can be robotically placed on strap substrate. In anotherembodiment, a strap assembly comprises an IC chip embedded in substrate,a portion of a dielectric layer overlying the strap substrate, and strapcontact pads on the dielectric layer. The embedded IC chip can bedeposited robotically (such as by “pick-and-place” methods) or byfluidic self-assembly methods, as described in U.S. Pat. Nos. 5,545,291and 7,260,882 (which are incorporated by reference herein for allpurposes). Pick-and-place techniques can be used to directly place asurface mounted IC on substrate 202 to form inlays without the use ofstrap assemblies.

In a specific embodiment, reader antenna 206 is both reader and readerantenna, such as an ALR-9650 offered by Alien. The ALR-9650 is a smartantenna, the reader electronics and a circularly polarized antennareside in a single package. Equally, a smart antenna can use a linearlypolarized antenna in lieu of a circularly polarized antenna. However,for many applications, the reader will be disposed away from its readerantenna(s) (e.g., one, two, three, four or more antennas). It should benoted that in embodiments where only one antenna is shown herein,alternative embodiments can include multiple antennas coupled to asingle reader to achieve greater diversity in signal orientation andcoverage.

Each inlay assembly is interrogated via reader antenna 206. For strapassembly testing, light source 208 singulates each strap assembly bymodifying a persistent node. The singulated strap assembly and readercan then exclusively communicate, allowing the reader to test theperformance of the specific strap assembly.

This test method eliminates the need for physical or electricalisolation (e.g., shielding) of individual inlay assemblies under test. Aresonant cavity, as used in U.S. patent application Ser. No. 11/809,610(which is incorporated by reference herein for all purposes) to receivea single response at the exclusion of others, is unnecessary. The readerneed not worry about interfering responses from adjacent inlayassemblies, since their persistent nodes would not satisfy the selectioncriteria. Light source 208 can then reposition its light beam 210 toanother strap assembly and repeat the process until all inlay assemblieson substrate 202 are tested. Inlay assemblies failing the performancetest can be marked by a laser or ink marker (not shown in FIG. 2) withrejection mark 212 or by entering a pass code in the tag's memory. Thistest method can also be applied to RFID straps and labels.

Substrate 202 can be a sheet, web (for example, an unwound roll in aroll-to-roll process), tape, reel, or the like. If substrate 202 isintermittently or continuously in motion relative to light source 208,then light source 208 can account for such motion in directing lightbeam 210 at its targeted strap assembly. For fast moving substrates,such as typically found in RFID roll-to-roll processing, two separatereaders or reader antennas with a light source between them may beneeded. The first reader energizes the inlays and sets at least onepersistent node to a first logic level for all strap assemblies. Theintervening light source illuminates a subset of strap assemblies (e.g.,one, two, three or more strap assemblies) to alter the persistent node.The second reader can then selectively communicate with the subset basedon the logical value of the persistent nodes. If more than one inlayassembly is illuminated, the reader may need to incorporateanti-collision algorithms for effective communication, such asimplemented for conventional EPC compliant tags.

FIG. 3 illustrates a simplified example of RFID system 300 according toan embodiment of the present invention. RFID system 300 includes atleast two readers. A first reader uses antenna 304 and light source 306to singulate, identify, and/or test inlay assemblies on substrate 302.Similarly, a second reader uses antenna 308 and light source 310 tosingulate, identify, and/or test inlay assemblies. By providing tworeaders, the throughput of system 300 can be doubled. Both the first andsecond readers can be controlled by the same host computer. To reducecontentions between the two readers, each can be assigned to use its owninventory session. For example, under the UHF Gen2 standard, tags havefour session flags, S0, S1, S2, and S3. Reader 1 can be dedicated to usethe S2 flag, while Reader 2 can be dedicated to use the S3 flag. Both S2and S3 flags are persistent.

FIG. 4 illustrates a simplified flowchart 400 for subset selectionaccording to an embodiment of the present invention. In step 402, areader transmits a continuous wave RF (radiofrequency) signal (“CW”) toprovide power to passive tags within the reader's field. The CW can be abroad beam signal encompassing a wide area. Next, in step 404, at leastone persistent flag is set. This can be accomplished by the readerissuing a SELECT command requiring all tags to assert the SL flag orother persistent flag (e.g., set one of S1, S2 or S3 flags to “B” logicvalue) after being inventoried. The reader then inventories all tags toensure each tag sets the designated persistent flag.

In step 406, the reader ceases transmissions for an off time. In oneembodiment, step 406 can be accomplished by frequency hopping inaccordance with 47 CFR Part 15, and more particularly subpart C. Duringat least a portion of the off time, a desired subset of tags isilluminated in step 408. The illumination with light can be narrowlyfocused, or a narrow beam in comparison to the CW RF signal. Forexample, tags can be illuminated for a fixed period of time, e.g., rangeof about 0.01 to about 0.5 seconds. The subset can include any number oftags, for example, one, two, three or more tags. In one embodiment, thesubset of illuminated tags is a proper subset of tags within thereader's RF field. That is to say, the illuminated tags are a subset ofthe tags within the reader's RF field, but the illuminated tags do notinclude all such tags.

The duration and intensity of illumination should allow a persistentflag to change value (for example, (i) SL to ˜SL or (ii) “B” value to“A” value for a session flag) before the completion of the off time. Instep 410, the reader resumes transmission of a continuous RF wave topower tags. The off time should be less than the minimum specifiedpersistent time (or alternatively, the minimum expected persistent timefor the particular tags) under non-illuminated conditions to ensure thatthe persistent flag correctly distinguishes the desired subset fromother tags.

The reader can now interrogate the desired subset in step 412. Forexample, using the ISO/IEC 18000-6c protocol, the reader can issue QUERYcommands indicating the persistent flag value as the criteria forparticipating in an inventory round. For instances, the QUERY commandcan specify deasserted SL flags (˜SL) in its “Sel” field, or S2 flagswith an “A” value. Accordingly, only tags matching the specifiedcriteria will respond to the reader in that inventory round.

FIG. 5 illustrates a simplified flowchart 500 for subset selectionaccording to another embodiment of the present invention. Flowchart 500includes several common elements to flowchart 400. Thus, given anunderstanding of the present invention from the detailed descriptionabove, flowchart 500 needs no further explanation.

Susceptibility to light can inexpensively enable another function for anRFID tag—light sensor. Susceptibility of persistent nodes or other tagfunction can be used as a light sensor to detect the presence of light,intensity of light, and/or the duration of illumination. By way ofexample, a reader can iteratively determine (by repetitive stepdecreases or increases in off times) the persistent time for one or moretags. If a persistent time is less than a nominal time or some minimumtime, the reader can infer or determine the presence and intensity oflight exposure on the tag. The reader can establish the approximate timeof day based on persistence timing (preferably, from results of aplurality of tags to provide superior accuracy).

As discussed above, Alien has devised circuitry to make persistent nodessubstantially immune to light. However, in order to achieve the benefitsdescribed herein, a tag manufacturer can elect to keep certainpersistent nodes susceptible and not others. For example, the persistentS2 and S3 flags under the UHF Gen2 standard can be made immune, whilekeeping the SL flag susceptible. In such case, embodiments of theinvention described herein that use the persistent S2 or S3 flags caninstead make use of the SL flag. In certain embodiments, an RFID tag canbe constructed, using either shielding or circuitry, so that somepersistent node or nodes are sensitive to light radiation and otherpersistent node (or nodes) are not sensitive to light radiation (or theyare much less sensitive to light radiation). Such an RFID tag may beconstructed using an embodiment of the light insensitive circuitrydescribed below.

The inventions have been generally described in detail for a subsetselection, but these inventions can be applied to other tag functions.For example, a persistent node can control write access to a tag'smemory, thereby only illuminated tags can be written to with newinformation. As another example, persistent nodes can control anoperational state of a tag—illumination of the tags causing the tag'sstate machine to move from a first operating state to a second operatingstate. These operating states can be a READY state, SLEEP state,ACKNOWLEDGED state, OPEN state, SECURED state, LOCKED state, REPLYstate, or the like. For tags implementing tiered identificationprotocol, such as described in U.S. Provisional Patent Application60/904,590 (which is incorporated herein for all purposes), illuminatedtags can respond with a first identifier, while non-illuminated tags canrespond with a second identifier. Light intensity can be used to affecta tag identification number (TID) of illuminated tags, including makingthe TID available or unavailable for reading and/or writing.

Additionally, the techniques described herein can be applied for thefollowing:

varying the intensity of light to develop range information;

light sources spaced at positions away from the reader to define zonesof interest or non-interest;

light sources used in a printer to do isolation allowing a cheaperprogramming head;

a varying light beam size coordinated with a positioning algorithm;

different and unique persistent nodes optimized for a particularwavelength for further isolation;

sweeping a light curtain in one two or three dimensions to helpcoordinate position;

using the same or a different light source to identify a particular tagone isolated; or

using the same light source that a barcode reader uses to read barcodes.

Light Insensitive Circuit Structures

Very low current circuits can undergo undesirable parametric shifts whenexposed to light, or more generally, electromagnetic radiation. It isnot always possible or economical to externally shield affectedcircuits. This disclosure pertains to integrated circuit structurescapable of reducing electromagnetic sensitivity of critical circuitparameters (e.g., analog data persistence time, time base oscillationfrequency, reference current magnitude, and other critical performanceparameters and functions).

Sufficiently energetic photons, upon entering a semiconductor, willcreate electron-hole pairs. Wavelength of these photons may be in thex-ray, ultraviolet, visible, or infrared portions of the electromagneticspectrum. Whether a particular wavelength poses circuit performanceproblems depends on the constitution, design, and packaging of thesemiconductor circuit. For the case of low-cost RFID, we areparticularly concerned about near-infrared radiation (700 nm to 1300 nm)in silicon circuitry, and specifically CMOS circuitry. It should berecognized; however, that other semiconductors and component types mightbe sensitive to different wavelength ranges. It is possible to blockincident radiation by the use of light shielding materials such a metal,thick paper or plastic, or pigments or dyes, but such light blockingmaterials or layers add undesirable cost or bulk to the RFID tag.

Focusing now on photo generated electron-hole pairs in silicon, theseexcess charge carriers diffuse throughout the silicon bulk until theyrecombine, or are separated by the electric field associated with a pnjunction. Carrier separation results in charge flow (current) that canperturb operation of low current circuits (nanoamps to picoamps).

FIG. 6 illustrates a schematic and cross sectional view of a circuitwith an analog storage device in accordance with one embodiment of thepresent invention. Analog storage occurs in the following manner. Then-channel access transistor is turned on by driving Strobe to anappropriately high, positive potential. Analog data is passed as avoltage from Data to Store. Strobe is then driven low, trapping chargeon the storage capacitor. This charge represents the analog voltage tobe “remembered”. Perturbation of trapped charge results when photogenerated electron-hole pairs are separated by the junction formedbetween p-substrate and n-channel drain. A curved arrow 620 denotescharge flow through this junction. The photo induced current removestrapped charge, changing the value of “remembered” voltage.

In one embodiment, the disclosed invention significantly reducesphotocurrent by preventing excess charge carriers from reaching anycritical junction. FIG. 7 illustrates a cross sectional view of acircuit structure having an n-channel transistor in accordance with oneembodiment of the present invention. A deep n-well is constructed tototally enclose the critical drain junction. Electron-hole pairsgenerated within p-well can still contribute to photocurrent dischargeof Store as illustrated with arrow 710, but since the volume of p-wellis quite small relative to volume of p-substrate, effective harmfulphotocurrent will be minimal. Electron-hole pairs generated withinp-substrate and deep n-well are rendered harmless in that any resultingcurrent exists in a ground loop 720.

The deep n-well implements a backside protection by controlling movementof excess charge carriers present in the silicon substrate. Care shouldbe taken to minimize photons directly entering p-well as no protectionis afforded against electron-hole pairs created in this region. Apractical step is to arrange topside metal layers in a manner thatprevents light from entering p-well. In this way the metal layerscombined with the deep n-well create a protective box around criticaljunctions.

It may appear that including deep n-well necessitates a semiconductorprocess modification. This is not necessarily the case. Many processtechnologies make possible an isolated p-well through incorporation of adeep n-well implant and judicious use of n-well.

FIG. 8 illustrates a cross sectional view of a circuit structure havingan isolated p-well in accordance with one embodiment of the presentinvention. A deep n-well is an implant peaked far below the siliconsurface and appears as a buried layer. Electrical connection is made viathe normal n-well.

The circuit of FIG. 6 has specific application in RFID chips forimplementing “persistent nodes”. Such nodes keep a temporary record ofchip state in an environment where brief power interruptions are acommon occurrence. A key requirement is that change of the stored analogvoltage not be too great over a time interval of seconds, independent ofsupply voltage. Clearly, uncontrolled photocurrents could result inrapid capacitor discharge. The disclosed invention provides a means forachieving proper persistent node operation in bright sunlightconditions, or in other situations where strong sources of infrared,visible, ultraviolet, or x-ray radiation are present.

In some embodiments, referring to FIG. 8, the specific electrodeconnections allow switch like behavior of the n-channel transistor. Itis permissible to disconnect deep n-well and p-well from Gnd andreconnect to Data. In this case, the MOS transistor exhibits a reducedthreshold voltage and functions in parallel with a parasitic, diodeconnected, NPN transistor. This scheme is advantageous in low voltageoperation of the analog storage device, specifically for persistent noderealization. The advantage derives from the fact that Strobe potentialrequired for passing data is lowered. A price to be paid is that Datamust be driven low when Strobe goes low in order that the NPN transistorbe turned off. In any event, the modified circuit remains photocurrentinsensitive regardless of power supply behavior.

Other variations can be envisioned if focus is shifted from analogstorage and trained solely on n-channel transistor function. Againreferring to FIG. 8, deep n-well can be disconnected from Gnd andreconnected to the most positive supply. The p-well can be connected toData. Grounding Data results in a common source configuration withlittle photocurrent mingling with drain (Store) output current. TreatingData as an output while connecting Store to the most positive supplycreates a source follower configuration in which little photocurrentcombines with source (Data) output current.

FIG. 9 illustrates a schematic and cross sectional view of a p-channelcircuit for Analog Data Storage in accordance with one embodiment of thepresent invention. FIG. 9 returns to the example of an analog storagedevice but this time the access transistor is p-channel. In oneembodiment, p-channel transistors can also benefit from the disclosedinvention although differences in details exist. Circuit operation is asfollows. The p-channel transistor is turned on by driving Strobe to anappropriately low potential. Analog data is passed as a voltage fromData to Store. Strobe is then driven high, trapping charge on thestorage capacitor. Perturbation of trapped charge results when photogenerated electron-hole pairs are separated, predominantly by thejunction formed between p-substrate and n-well. A curved arrow 910denotes charge flow through this junction. The photo induced currentremoves trapped charge, changing the value of “remembered” voltage.

FIG. 10 illustrates a cross sectional view of a circuit structure withan PMOS transistor in accordance with one embodiment. A deep n-wellcompletely encloses the critical n-well/p-well junction. P-well volumeis favorably smaller than p-substrate volume leading to greatly reducedphotocurrent. As in the n-channel case, deep n-well acts to controlmovement of photo induced excess charge carriers, forcing these carriersto become part of a ground loop current 1020. Also, various alternateelectrode connections can be envisioned that preserve the invention'sadvantage of photo insensitivity. These alternate connections may beapplied to the analog storage device or to general operation of ap-channel transistor.

Two significant departures exist relative to the previously explainedn-channel implementation. Isolated p-well cannot be achieved in thesame, simple manner. The reason is deep n-well must now occur belownormal n-well, which is an integral part of a p-channel transistor. Amodified, non-standard, process technology is necessitated. Themodifications are not substantial, but are nonetheless, modifications.Clearly, the requisite technology accommodating p-channel transistorswould also suffice for their n-channel counterparts (compare FIGS. 7 &10).

The second departure stems from the fact that a p-channel transistor isafforded natural immunity to photocurrents under specific biasconditions. Looking back at FIG. 9, if a positive power supply isavailable, then n-well can be disconnected from Store and reconnected topositive supply. Most photocurrent will now be shunted away from Storein the direction of positive supply. A small photocurrent will continueto affect trapped charge on Store. Magnitude of this current will begreatly reduced because the volume of n-well is so much less than volumeof p-substrate. This arrangement works well for a wide variety ofcircuits and does away with the necessity of deep n-well as a means toreduce photo sensitivity. Unfortunately, the persistent node applicationin RFID chips is an example where no convenient, reliable supply isavailable.

All of the foregoing is quite general with regard to reducing photosensitivity of CMOS transistors and is extendable to a wide variety ofCMOS circuits. As an example of this generality, consider the currentbiased oscillator of FIG. 11 in accordance with one embodiment.Transistors P1, P2, N1, and N2 in concert with resistor R establish abias current I. This current is mirrored in devices P3, P4, and P5. Voutis an oscillating voltage with frequency inversely proportional to thevalue of I. Photo insensitive oscillation occurs when drain currents forall transistors are relatively insensitive to illumination. Of course,this can be achieved by running an arbitrarily high bias current.Alternatively, bias current can be kept advantageously low andphotocurrents managed in accord with the disclosed invention. Specificoperations to be taken are:

-   -   Place all p-channel devices in a common n-well keeping n-well        area as small as possible. Connect n-well to Vdd. No deep n-well        is required, as the normal n-well will shunt majority of        photocurrent to Vdd. Use top-level metal layers to block light        from n-well. This is particularly critical in the vicinity of        any p-channel drain to n-well junction. It is preferable to        completely shield n-well.    -   Place all n-channel devices in a common, isolated p-well keeping        p-well area as small as practical. Connect p-well to Gnd. A deep        n-well connected to Gnd is necessary to act as shunt for        photocurrents. Use top level metal layers to block light from        p-well. This is particularly critical in the vicinity of any        n-channel drain to p-well junction. It is preferable to        completely shield isolated p-well.

In some embodiments, the light insensitive circuit structures describedherein provide the following:

-   -   1. Circuit structure in which isolated p-well is used to reduce        photocurrent present in terminal currents of CMOS transistors        (FIG. 10).    -   2. Circuit structure in which isolated p-well is used to reduce        photocurrent present in terminal currents of n-channel        transistors and where p-well construction is consistent with        commonly available semiconductor process technology (FIG. 8).    -   3. Specific application of persistent nodes in RFID chips    -   N-channel version (FIG. 7)        -   P-channel version (FIG. 10)        -   CMOS version (FIGS. 7 & 10 combined)    -   4. General application of low current circuits in RFID system        (FIG. 11). In other embodiments, additional ways to enhance        light insensitivity include the inclusion of backside        metallization (backside of chip), the creation of traps and/or        recombination sites in a substrate material, and/or the creation        of a dielectric layer within the substrate material, removal of        the substrate, or using an insulating substrate.

Transistor Based Voltage Multipliers and Demodulators

In some embodiments, MOS transistor devices are used as rectifyingelements in voltage multipliers and demodulators. A special bias circuitreduces the effective threshold of the rectifying elements, allowing theuse of normal threshold transistors.

Voltage multipliers that are powered by AC input voltages findapplications in many areas. The basic building block of such a circuitis usually a voltage doubler circuit containing two rectifying elementsand producing a DC output voltage. A multiple of such circuits can bearranged with the AC input voltages in parallel and the DC outputvoltages in series to produce a multiple of the dc voltage for a givenAC input voltage. This arrangement forms the voltage multiplier. Intheory, an arbitrarily high total DC voltage can be produced dependingon the number of stages.

The rectifying elements that make up the doubler circuit can be one of anumber of devices. In most instances, doubler performance is enhanced bya lower turn-on threshold of the rectifying element, especially at lowAC input voltages. Diodes and diode-connected transistors are the mostcommon element used in integrated circuit implementations. For highspeed operation at about 1 GHz and beyond, Schottky diodes are oftenused. These diodes are fast due to the conduction mechanism, and have amuch smaller threshold voltage for forward conduction. However,optimized Schottky diodes are not always available in semiconductorprocesses, and may require additional processes and masking steps. Thediode-connected MOS transistor device is available as a standard part ofprocesses, but in an unmodified form usually has a higher thresholdvoltage than a Schottky diode.

In certain embodiments of the present invention, standard high-thresholdMOS devices are used to implement a high performance voltage multiplieror doubler circuit, without the use of any special processes or device.Doubler and voltage multiplier circuits have been constructed and testedwhich out-perform Schottky diodes at 915 MHz. A special version of thecircuit has also been designed for applications where a more linearresponse to the input AC voltage is required, such as demodulators. Theapplication in which the circuit was tested was a Radio FrequencyIdentification device, or RFID.

The basis of the circuit is the use of a very low power circuit togenerate a bias voltage. This bias voltage is used to effectively reducethe threshold voltage of a MOS transistor. This device then behaves in amanner similar to a low-threshold diode connected transistor. The resultis a high performance doubler or multiplier circuit using standard CMOSprocessing.

FIG. 12 illustrates a conventional MOS transistor doubler circuitimplementation in accordance with a prior approach. The circuit 1200operates with the normal threshold of the MOS devices and requires alarge AC input voltage (identified as RF) to operate well. A first stage1210 illustrates a CMOS implementation and a second stage 1220illustrates a NMOS implementation.

FIGS. 13A and 13B illustrate CMOS transistor doubler circuitimplementations in accordance with certain embodiments of the presentinvention. The circuit 1340 illustrated in FIG. 13A shows an example ofcross coupling the gates of the devices to reduce the threshold voltageof the devices by approximately the DC output voltage. However, thiscircuit was shown in simulations to suffer from a very long start-uptime, i.e., the circuit at initial DC voltage of zero volts has nothreshold voltage reduction, and therefore has low performance until asignificant DC voltage is produced.

The circuit 1370 illustrated in FIG. 13B shows a solution to thisproblem. Two separate bias circuits 1380 and 1390 are implemented witheach bias circuit supplying a gate to source bias, which is independentof the DC output voltage. The bias circuits are similar to the circuit1340 described above. However, since the bias circuits effectively haveno load, they can achieve start-up in a very short time and are notaffected by current supplied to the DC output voltage. In addition,since the bias circuit has no load (only leakage and sub-thresholdcurrents), it can produce a higher bias for a given RF input voltagethan a loaded version of the circuit 1340, and the rectifying elements1392 and 1394 operate with a lower effective threshold than with thecircuit 1340.

The circuits 1340 and 1370 take advantage of the complementary nature ofthe N and P transistors to produce a circuit in which no RF voltage isdirectly applied to a transistor gate.

FIG. 14A illustrates a block diagram of a circuit 1400 that receives aRF input source and generates a DC output voltage in accordance withsome embodiments of the present invention. The circuit 1400 includes afirst bias circuit 1402 to supply a gate to source bias 1403, which isindependent of a direct current (DC) output voltage. The circuit 1400further includes a voltage multiplier circuit 1410 coupled to the firstbias circuit 1402. The voltage multiplier circuit 1410 has two or moremetal-oxide-semiconductor (MOS) transistors. The voltage multipliercircuit 1410 generates the DC output voltage for powering a RFidentification tag in accordance with one embodiment. The first biascircuit 1402 receives the RF input source and generates the gate tosource bias 1403 for a gate terminal of one of the MOS transistors. Asecond bias circuit 1404 receives the RF input source and generates agate to source bias 1405 for a gate terminal of another one of the MOStransistors.

FIG. 14B shows a schematic of a CMOS voltage multiplier circuit inaccordance with one embodiment of the present invention. The voltagemultiplier circuit 1420 may be fabricated with standard CMOS processing(e.g., 0.16 micron CMOS process) and used as the stages of a multi-stagevoltage multiplier (e.g., 6 stage voltage multiplier illustrated in FIG.16). Multiple types of capacitors are used to increase capacitancedensity, and the horizontal capacitor shown across the DC output voltagein FIG. 13B is replaced with a device (e.g., C3, M0) to ground.

In one embodiment, this circuit 1420 was found in simulations to be ableto supply a voltage of 0.7 volts at a micro amp of current with an inputRF voltage at the inputs of the circuit of less than 190 mv.Implementations of the circuit in a CMOS process verified thatperformance.

This circuit 1420 was found in simulations and implementation to have acompressing effect as the RF input voltage is increased. The DC outputvoltage can approach a plateau or even decrease as the RF input voltageis increased. The cause of this effect is that the bias voltage becomeshigh enough to produce operating regions of large reverse current in therectifying elements (e.g., NM2, PM2). Careful design can prevent thiseffect from being a problem in a voltage multiplier used to provide apower supply, where the minimum and maximum required voltages are fairlyclose together. In an application where more linearity is required, thecircuit needs to be modified.

The circuit 1420 includes a bias circuit 1430 to supply a gate to sourcebias 1432, which is independent of a direct current (DC) output voltage1424. The circuit 1420 further includes a voltage multiplier circuit1440 that is coupled to the separate bias circuit 1430. In oneembodiment, the voltage multiplier circuit 1440 has at least onen-channel metal-oxide-semiconductor (NMOS) transistor (e.g., NM2) and atleast one p-channel metal-oxide-semiconductor (PMOS) transistor (e.g.,PM2) and also includes C0. The voltage multiplier circuit 1440 generatesthe DC output voltage for powering a RF identification tag in accordancewith one embodiment. The bias circuit 1430 receives the RF input sourceand generates the gate to source bias 1432 for a gate terminal of one ofthe MOS transistors (e.g., NM2). A second bias circuit 1450, separatefrom the circuit 1440, receives the RF input source and generates a gateto source bias 1452 for a gate terminal of one of the MOS transistors(e.g., PM2).

The bias circuit 1430 generates the gate to source bias 1432 for a gateterminal of one of the NMOS transistors to DC bias the NMOS transistor(e.g., NM2) within a certain range (e.g., approximately 100 to 200milliVolts) of a threshold voltage of the NMOS transistor orsubstantially to the threshold voltage. In one embodiment, the biascircuit 1430 is coupled in series between a gate of one of the NMOStransistors (e.g., NM2) of the voltage multiplier circuit 1440 and a DCvoltage input (e.g., dcin 1422) to the voltage multiplier circuit 1440.The bias circuit 1450 generates a gate to source bias 1452 for a gateterminal of one of the PMOS transistors (e.g., PM2) to DC bias the PMOStransistor within a certain range of a threshold voltage of the PMOStransistor or substantially to the threshold voltage.

In one embodiment, the bias circuit 1430 includes NM1, PM0, C1, C2, andM1. A gate terminal of the NMOS transistor (e.g., NM1) of the biascircuit 1430 is coupled to a source terminal of the PMOS transistor(e.g., PM0) of the bias circuit 1430 and a gate terminal of the PMOStransistor is coupled to a source terminal of the NMOS transistor inorder to lower threshold voltages of the NMOS and PMOS transistors. TheRF input source is not directly applied to any of the gate terminals ofthe MOS transistors in the bias circuits or in the voltage multipliercircuit.

In another embodiment, the voltage multiplier circuit is implementedwith NMOS transistors rather than NMOS and PMOS transistors asillustrated in FIG. 14B. In yet another embodiment, the voltagemultiplier circuit is implementing with PMOS transistors.

FIG. 15 shows a schematic of a doubler circuit suitable for use as ademodulator in accordance with one embodiment of the present invention.The main difference between the circuit 1520 and the circuits 1400 or1420 is that a clamping transistor device (e.g., M120, M7) is used tolimit the value of the bias voltage to approximately one transistorthreshold or below. Additional regulation is performed on the biasvoltage applied to the rectifying elements (e.g., M17, M15) by the useof an RF-operated isolation switch (e.g., M118, M121). Even with theclamp device, the bias voltage is not well regulated when the RF inputvoltage is decreased, and the clamped voltage will decrease somewhatduring low excursions (as during modulation) of the RF input voltage.The isolation switch ensures that the rectifier biases are not pulleddown during the low RF input voltage excursions. In simulations, thiscircuit 1520 provides much more linear demodulation and fewer artifactsthan the circuits 1400 or 1420 used in the power supply voltagemultiplier.

In one embodiment, a demodulator circuit 1520 includes a bias circuit1530 to supply a gate to source bias 1532 to a voltage multipliercircuit 1540. The circuit 1520 also includes a clamping transistor(e.g., M120) coupled to the bias circuit 1530. The circuit 1520 alsoincludes the voltage multiplier circuit 1540 that is coupled to theclamping transistor. The voltage multiplier circuit 1540 has at leastone n-channel metal-oxide-semiconductor (NMOS) transistor (e.g., M17)and/or at least one p-channel metal-oxide-semiconductor (PMOS)transistor (e.g., M15). The voltage multiplier circuit 1540 generates ademodulated output signal 1524 that demodulates information carried by aRF input signal. The bias circuit 1530 receives the RF input signal andgenerates the gate to source bias 1532 for a gate terminal of one of theMOS transistors (e.g., M17). The clamping transistor (e.g., M120) limitsa value of the gate to source bias to approximately one thresholdvoltage of the clamping transistor or less. The bias circuit 1530generates the gate to source bias for a gate terminal of one of the NMOStransistors to DC bias the NMOS transistor (e.g., M17) within a certainrange (e.g., approximately 100 to 200 milliVolts) of a threshold voltageof the NMOS transistor or substantially to the threshold voltage. Thebias circuit 1530 is coupled in series between a gate of one of the NMOStransistors and a DC voltage input 1522 to the voltage multipliercircuit 1540.

The circuit 1520 also includes a bias circuit 1550 to receive the RFinput source and generate a gate to source bias for a gate terminal ofone of the MOS transistors. A clamping transistor (e.g., M7) is coupledto the bias circuit 1550. The clamping transistor limits a value of thegate to source bias to approximately one threshold voltage of the secondclamping transistor or less.

The circuit 1520 also includes an isolation switch (e.g., M118) that iscoupled to the bias circuit 1530. The isolation switch regulates thevoltage bias generated by the bias circuit 1530. In a similar manner, anisolation switch (e.g., M121) regulates the voltage bias generated bythe bias circuit 1550.

In another embodiment, the circuit 1520 is altered to be implementedwith merely NMOS transistors rather than NMOS and PMOS transistors asillustrated in FIG. 15. In yet another embodiment, the circuit 1520 isaltered to be implemented with merely PMOS transistors.

In certain embodiments, the circuit 1520 is altered to enhance the lightinsensitivity of the circuit. Body contacts of PM10 and M118 can beconnected to dcout and the body contacts of M16 and M121 can beconnected to dcin.

FIG. 16 shows a schematic of a CMOS multi-stage voltage multipliercircuit in accordance with one embodiment of the present invention. Themulti-stage voltage multiplier circuit 1600 may be fabricated withstandard CMOS processing (e.g., 0.16 micron CMOS process). In certainembodiments, each stage 1610, 1620, 1630, 1640, 1650, and 1660 of themulti-stage circuit 1600 is one of the voltage multiplier circuit 1400,1420, or an alternative implementation such as the circuit 1700 to bediscussed below or a NMOS implementation or a PMOS implementation. Thedetector circuit 1602 may be implemented with the circuit 1520 discussedabove or a NMOS implementation or a PMOS implementation.

In one embodiment, the multi-stage voltage multiplier circuit 1600includes a first stage 1610 of the multi-stage voltage multipliercircuit. This stage includes a bias circuit (e.g., 1402, 1430) having noeffective load, a voltage multiplier circuit (e.g., 1410, 1440) that iscoupled to the bias circuit, and another bias circuit (e.g., 1404, 1450)that is also coupled to the voltage multiplier circuit. The stage 1610includes similar components and functionality as described above inconjunction with the circuits 1400 and 1420 or described below inconjunction with FIG. 17A.

A second stage 1620 and subsequent stages may include similar componentsand functionality as described above in conjunction with the circuits1400 and 1420. Alternatively, the second stage 1620 and subsequentstages may include an alternative implementation of the circuits 1400and 1420 as illustrated in FIG. 17B and described below.

In one embodiment, the circuit shown in FIG. 17A shows a preferredimplementation of this invention where the light insensitivity of themultiplier is improved by tying the body and isolation wells of themultiplier to low impedance nodes of the power multiplier, rather thanthe high impedance nodes at the gates of the power multiplier. The bodyand isolation well of the NFET (e.g., NM32) is tied to the higher DC outnode of the doubler, and the body of the PFET (e.g., PM32) is tied tothe lower DC in node in order to use the back gate effect to lower thethreshold of the devices.

FIG. 17A shows a schematic of a CMOS voltage multiplier circuit inaccordance with another embodiment of the present invention. The voltagemultiplier circuit 1700 may be fabricated with standard CMOS processing(e.g., 0.16 micron CMOS process) and used as one or more stages of amulti-stage voltage multiplier (e.g., circuit 1600). In an embodiment,the circuit 1700 is used for the first stage 1610. The circuit 1700includes similar devices and functionality in comparison to the circuits1400 and 1420. However, the body contacts of some of the MOS devices inFIG. 17A are connected to low impedance nodes of the power multiplier,rather than the high impedance nodes at the gates of the powermultiplier to improve the light insensitivity of the multiplier. Forexample, the body contacts of PM30, M310, and PM32 are connected to dcinand the body contact of NM32 is connected to dcout.

The circuit 1700 includes antbst_in 1736, antenna 1734, antbst_out 1738,gate to source bias 1742, gate to source bias 1732, dcin 1722, and dcout1724.

In another embodiment, the circuit 1700 is altered to be implementedwith merely NMOS transistors rather than NMOS and PMOS transistors asillustrated in FIG. 17A. In yet another embodiment, the circuit 1700 isaltered to be implemented with merely PMOS transistors.

FIG. 17B shows a schematic of a CMOS voltage multiplier circuit inaccordance with another embodiment of the present invention. The voltagemultiplier circuit 1700 may be fabricated with standard CMOS processing(e.g., 0.16 micron CMOS process) and used as one or more stages of amulti-stage voltage multiplier (e.g., circuit 1600). In an embodiment,the circuit 1780 is used for the second stage 1620 and subsequentstages. The circuit 1780 includes similar devices and functionality incomparison to the circuits 1400 and 1420. However, the second biascircuit (e.g., 1404, 1450) from the circuits 1400 and 1420,respectively, is removed or disconnected from the circuit 1780. The biasfor the gate terminal of the PMOS transistor (e.g., PM22) is connectedto ground for the second stage 1620 as illustrated in FIG. 16. The biasfor the gate terminal of the PMOS transistor (e.g., PM22) is connectedto an intermediate voltage output node (e.g., dc1, dc2, dc3, dc4) forsubsequent stages 1630, 1640, 1650, 1660, respectively, as illustratedin FIG. 16. The DC output of a stage (e.g., 1610, 1620, 1630, 1640,1650) is feed as a DC input for a subsequent stage (e.g., 1620, 1630,1640, 1650, 1660). Stage 1660 generates the DC output used to supplypower for a chip (e.g., RF ID tag). In other embodiments, fewer or morestages can be combined for multiplying voltage and generating a DCoutput voltage for a RF ID tag.

In another embodiment, the circuit 1780 is altered to be implementedwith merely NMOS transistors rather than NMOS and PMOS transistors asillustrated in FIG. 17B. In yet another embodiment, the circuit 1780 isaltered to be implemented with merely PMOS transistors.

Temperature Sensing RFID Tag

In one embodiment, an RFID tag incorporates an oscillator whosefrequency varies with temperature. The oscillation frequency of the tagis used to infer the approximate temperature of the tag. A passive RFIDtag incorporates an oscillator whose frequency varies with temperature.The oscillation frequency of the tag is used to infer the approximatetemperature of the tag. A RFID tag incorporates an oscillator whosefrequency varies. The oscillation frequency of the tag is used to infersome physical property of the tag or of the environment that the tag isin, or of some input to the tag.

An RFID tag with an oscillator whose frequency varies with temperatureis compared to the frequency of another oscillator at the interrogator.Calibration information regarding the frequency of the oscillator at thetag is stored, and then used together with the oscillation frequency ofthe tag at a time, to infer the approximate temperature of the tag.

An RFID tag with an oscillator whose frequency varies with temperature,and whose frequency does not vary substantially with light, has anoscillation frequency which is compared to another oscillator at theinterrogator. Calibration information regarding the frequency of theoscillator at the tag is stored, and then used together with theoscillation frequency of the tag at a time, to infer the approximatetemperature of the tag.

An RFID tag includes a tag oscillator, and the tag starts countingoscillations beginning at a specified time, and then stops counting atanother specified time, and the number of oscillations is returned tothe interrogator. The interrogator then uses the number of oscillationsreturned, along with the calibration information to infer thetemperature of the tag.

A reader sends a first calibration time marker to a tag, such as therising edge of an amplitude modulated signal; and then a secondcalibration time mark to the tag, such as the rising edge of anotheramplitude modulated signal, and the tag stores a number corresponding tothe amount of time between the two calibration time markers into aregister, for return to the reader when a command is sent to retrieveinformation from the tag.

A reader sends a first calibration time marker to a tag, such as therising edge of an amplitude modulated signal; and then a secondcalibration time mark to the tag, such as the rising edge of anotheramplitude modulated signal, and the tag stores a number corresponding tothe amount of time between the two calibration time markers into aregister which is mapped into the tag memory map.

Calibration information regarding the frequency of an oscillator at thetag is stored in the tag long term memory. Calibration informationregarding the frequency of an oscillator at the tag under variousconditions is stored in the tag long term memory. Calibrationinformation regarding the frequency of an oscillator at the tag atdifferent temperatures is stored in the tag long term memory.

As an example of an oscillator which varies with temperature, considerthe current biased oscillator of FIG. 11. Transistors P1, P2, N1, and N2in concert with resistor R establish a bias current I. This current ismirrored in devices P3, P4, and P5. Vout is an oscillating voltage withfrequency inversely proportional to the value of I. If the resistor R isimplemented with a polysilicon resistor, then its resistivity goes downas its temperature increases. The oscillation frequency then goes upmonotonically as the temperature of the RFID tag increases.

In some embodiments, RF systems include RF ID readers and tags that canbe used for various applications, such as described in U.S. ProvisionalPatent Application 60/904,590 (which is incorporated herein for allpurposes).

FIG. 18 illustrates an exemplary RFID system 1800 according to anembodiment of the present invention. System 1800 includes a reader 1802coupled to antennas 1804 a, 1804 b and computer 1806. Reader 1802 can beoperating in a bistatic, monostatic, or multistatic mode with theantennas. As illustrated in FIG. 18, tags 1808 a, 1808 b are physicallycoupled to items to be identified. These items are moved along aconveyer and recycle codes read by reader 1802. This recycle informationis provided to automatic sorter 1810. Sorter 1810 can segregate itemsbased on their respective recycle information using magnets, sifters,centrifuges, fluid separators, vacuum loaders, or other known techniquesto divert items (or a constituent material, gas, liquid, or sludge) toan appropriate branch line, bin, receptacle, compactor, or hopper.Automatic sorting can provide greater efficiency over manual sortingrequiring visual inspection of items. It can also reduce or eliminatethe need for private consumers to pre-sort their refuse beforecollection. Although FIG. 18 depicts a distributed system 1800, analternative system can include a reader, antennas, computer, andautomatic sorter integrated into a single piece of equipment. In aspecific embodiment, system 1800 can also include decontaminationequipment (e.g., to wash, heat, or sterilize) and containment equipmentfor hazardous waste. Decontaminants can include alcohol solution,ethylene oxide, water, detergent, hydrogen peroxide, sodium hydroxide,chloramines solutions, hot steam, hot air stream, and the like.

FIG. 19 illustrates an exemplary radio frequency identification (RFID)system 1900, which includes an RFID reader 1901 and a plurality of RFIDtags 1931, 1933, 1935, . . . , and 1939. The system can be either areader-talks-first or tag-talks-first system using passive,semi-passive, or active tags. Reader 1901 typically includes a receiver1919 and a transmitter 1923 (alternatively, a transceiver), each ofwhich is coupled to an I/O (input/output) controller 1917. The receiver1919 may have its own antenna 1921, and the transmitter 1923 may haveits own antenna 1925. It will be appreciated by those in the art thatthe transmitter 1923 and the receiver 1919 may share the same antennaprovided that there is a receive/transmit switch which controls thesignal present on the antenna and which isolates the receiver andtransmitter from each other. The receiver 1919 and the transmitter 1923may be similar to receiver and transmitter units found in conventionalreaders. In North America, the receiver and transmitter for RFIDtypically operate in a frequency range of about 915 megahertz (e.g., 902MHz-928 MHz) using spread spectrum techniques (e.g., frequency hopping).In Europe, the frequency range is about 866 megahertz (e.g., 865.7MHz-867.7 MHz). Other regions have set aside, or are in the process ofsetting aside, frequency ranges for operation—these ranges of operationtypically lie somewhere in the overall range of 200 MHz to 5 GHz. Eachis coupled to the I/O controller 1917 which controls the receipt of datafrom the receiver and the transmission of data, such as commands, fromthe transmitter 1923. The I/O controller is coupled to a bus 1915 whichis in turn coupled to a microprocessor 1913 and a memory 1911.

There are various different possible implementations for the processingsystem represented by elements 1911, 1913, 1915, and 1917, which may beused, for example, in the exemplary RFID reader 1901 of FIG. 19. In oneembodiment, the microprocessor 1913 is a programmable microcontroller,such as an 8051 microcontroller or other well-known microcontrollers ormicroprocessors (e.g. a PowerPC microprocessor) and the memory 1911includes dynamic random access (DRAM) memory. Memory 1911 may alsoinclude a non-volatile read only memory for storing data and softwareprograms. The memory 1911 typically contains a program which controlsthe operation of the microprocessor 1913 and also contains data usedduring the processing of tags as in the interrogation of tags. In someembodiments of the present invention, the memory 1911 would typicallyinclude a computer program which causes the microprocessor 1913 todecode received tag data with the appropriate tag-to-reader protocolscheme. The reader 1901 may also include a network interface (not shownin Fig.), such as an Ethernet interface, universal bus interface, orWi-Fi interface (such as IEEE 802.11, 802.11a, 802.11b, 802.16a,Bluetooth, Proxim's OpenAir, HomeRF, HiperLAN and others), which allowsthe reader to communicate to other processing systems through a network,including without limitation an inventory management system, centralstore computer, personal computer, or database server. The networkinterface would typically be coupled to the bus 1915 so that it canreceive data, such as the list of tags identified in an interrogation,from either the microprocessor 1913 or from the memory 1911.

FIG. 20 shows an example of one implementation of a radio frequencyidentification (RFID) tag which may be used with the present invention.The tag 2000 includes an antenna 2010 (alternatively, two, three or moreantennas) which is coupled to a receive/transmit switch 2030. Thisswitch is coupled to the receiver and demodulator 2050 and to thetransmitter and modulator 2090. A controller unit 2070 is also coupledto the receiver/demodulator 2050 and to the transmitter/modulator 2090.The particular exemplary RFID tag shown in FIG. 20 may be used invarious embodiments of the present invention in which data is maintainedin a memory (not shown in FIG. 20). The receiver and demodulator 2050receives signals through the antenna 2010, e.g., interrogation signalsfrom a reader (not shown in FIG. 20), and the switch 2030 receives anddemodulates the signals and provides these signals to the controllerunit 2070. Commands received by the receiver 2050 are passed to thecontroller of the unit 2070 in order to control the operation of thetag. Any additional data received by the receiver 2050 is also passed tothe control unit 2070, and this data may include handshaking data (e.g.,parameters for a tag-to-reader encoding protocol). The transmitter andmodulator 2090, under control of the control unit 2070, transmitsresponses to the commands or other processed data through the switch2030 and the antenna 2010 to the reader. It will be appreciated by thoseskilled in the art that the transmitter may be merely a switch or otherdevice which modulates reflections from an antenna, such as antenna2010.

In certain embodiments of the present invention, RFID tags may bedesigned with a small integrated circuit (IC) area, a small memory,atomic transactions to minimize tag state storage requirements, and thelike. This type of design will lower the tag production cost, therebyenabling wide-scale adoption of RFID labeling in a variety ofindustries, for example, in the supply chain.

FIG. 21 shows an example of a low cost tag 2100. The tag 2100 includesan antenna 2101 and an integrated circuit (IC) 2103, coupled together.Tag IC 2103 implements the command protocol and contains data such as anEPC. The antenna 2101 receives the reader-generated interrogationsignals and reflects the interrogation signal back to the reader inresponse to a modulation signal created by the tag IC 2103. Theexemplary tag IC 2103 comprises a radio frequency (RF) interface andpower supply 2111 (e.g., circuits 1400, 1420, 1700, stages from circuit1600), data detector (e.g., circuit 1520) and timing circuit 2113,command and control 2115, data modulator 2117, and memory 2119. In oneembodiment, command and control 2115 may include static logic (such as astate machine) which implements communication protocols according tovarious embodiments of the present invention.

The RF interface and power supply 2111 converts the RF energy into theDC power required for the tag IC 2103 to operate and provides modulationinformation to the data detector and timing circuit 2113. The RFinterface also provides a means of coupling the tag modulation signalsto the antenna for transmission to the reader. The data detector andtiming circuit 2113 demodulates the reader signals and may generatetiming and data signals used by the command and control 2115, includinga subcarrier sequence. The command and control 2115 coordinates all ofthe functions of the tag IC 2103. The command and control 2115 mayinclude state logic to interpret data from the reader, perform therequired internal operations, and determine if and/or how the tag willrespond to the reader. The memory 2119 contains the EPC, which may beassociated with the tagged item. The data modulator 2117 translates thebinary tag data into a tag-to-reader encoded signal that is then appliedto the RF interface 2111 and transmitted to the reader (e.g., reader 501of FIG. 5). In one embodiment, IC 2103 is a NanoBlock™ IC made by AlienTechnology Corporation of Morgan Hill, Calif.

The design and implementation of RFID tags can be characterized in termsof layers. For example, a physical and environmental layer characterizesthe mechanical, environmental, reliability and manufacturing aspects ofa tag, an RF transport layer characterizes RF coupling between readerand tag, and a communication layer characterizes communications/dataprotocols between readers and tags. Various different implementations oftags at different layers can be used with embodiments of the presentinvention. It is understood that the implementations of the tags are notlimited to the examples shown in this description. Different tags orcommunication devices can use methods and apparatuses of the embodimentsof the present invention for communication according to the needs of theparticular application.

In one embodiment of the present invention, a tag may be fabricatedthrough a fluidic self-assembly process. For example, an integratedcircuit (e.g., 2103 of FIG. 21) may be fabricated with a plurality ofother integrated circuits in a semiconductor wafer. The integratedcircuit will include, if possible, all the necessary logic of aparticular tag, possibly excluding the antenna 2101. Thus, all the logicshown in the tag 2100 would be included on a single integrated circuitand fabricated with similar integrated circuits on a singlesemiconductor wafer. Each circuit may be programmed (or pre-programmed)with a unique identification code and then singulated (and shaped) fromthe wafer. Integrated circuit block can be singulated by manytechniques, including those described in U.S. patent application Ser.No. 11/546,683 filed on Oct. 11, 2006, entitled “Block FormationProcess”, which is incorporated by referenced. Integrated circuit blocksare next suspended in a fluid. The fluid is then dispersed over asubstrate, such as a flexible substrate, to create separate tags.Receptor regions in the substrate would receive at least one integratedcircuit, which then can be connected with an antenna on the substrate toform a tag. An example of fluidic self-assembly (FSA) is described inU.S. Pat. No. 6,864,570, entitled “Method for FabricatingSelf-Assembling Microstructures,” which is incorporated by referenceherein.

Alternatively, other conventional or unconventional assembly methods maybe used to construct the radio frequency tag. Silicon integratedcircuits, formed using standard CMOS processes can be bonded to anantenna using robotic techniques (e.g., pick and place methods, surfacemounted flip chips, and the like), vibratory assembly techniques, or awire bonding construction. The chip can be placed in a carrier, such asa lead frame or a strap, or be bonded directly to an antenna. Strapattachment may be accomplished in automatic web processes using AlienTechnology Corporation's high speed strap attach machine (HiSAM™machine). The chip need not be made of silicon—devices built fromsemiconductors such as GaAs, or even organic semiconductors, can achievethe benefits derived from these communication methods.

In the foregoing specification, the invention has been described withreference to specific exemplary embodiments thereof. It will be evidentthat various modifications may be made thereto without departing fromthe broader spirit and scope of the invention as set forth in thefollowing claims. The specification and drawings are, accordingly, to beregarded in an illustrative sense rather than a restrictive sense.

What is claimed is:
 1. A radio frequency identification (RFID) systemcomprising: a tag with an integrated circuit having at least one flag,the at least one flag having a persistence time reduced by exposure tolight; an RFID reader; and a light source configured to reduce thepersistence time.
 2. The RFID system of claim 1 wherein the light sourceis a localized light source.
 3. The RFID system of claim 1 wherein theflag is a logic value indicating a tag selection state.
 4. The RFIDsystem of claim 1 wherein the light source produces infraredelectromagnetic waves.
 5. The RFID system of claim 1 wherein the lightsource is an infrared light emitting diode.
 6. The RFID system of claim1 wherein the RFID reader is a handheld reader.
 7. The RFID system ofclaim 1 wherein the RFID reader is configured to receive backscattermodulated data from the tag within a band of 860 MHz to 960 MHz.
 8. TheRFID system of claim 1 wherein the RFID reader is configured to receivedata from the tag within a band of 13.56 MHz±10%.
 9. The RFID system ofclaim 1 wherein the integrated circuit includes a second flag having apersistence time unaffected by the exposure to light.
 10. The RFIDsystem of claim 9 wherein a second flag's persistence time is reduced bya frequency differing from the exposure.
 11. A radio frequencyidentification (RFID) reader comprising: a radio frequency source; anantenna to transmit radio frequency waves generated by the radiofrequency source; and a light source to transmit light waves thatmanipulate a persistence time of an RFID tag, wherein the antennatransmits radio frequency waves to a first area and the light sourcetransmits to a second area, the second area being a proper subset of thefirst area.
 12. The RFID reader of claim wherein the light source is aninfrared source and the light waves include infrared electromagneticwaves.
 13. A method for selecting a subset of radio frequencyidentification (RFID) tags comprising: setting a persistent node foreach of the plurality of passive tags to a first logic value, the nodecapable of maintaining the first logic value for a first time period inthe absence of power; interrupting power to the passive tag for a secondtime period, the second time period being shorter than the first timeperiod; and illuminating a subset of the plurality of tags with lightduring at least a portion of the second time period, persistent nodes ofthe subset degenerating to a second logic value during the second timeperiod.
 14. The method of claim 13 further comprising: querying, afterthe second time period, passive tags having nodes with a first logicvalue, but not passive tags having nodes with a second logic value. 15.The method of claim 13 further comprising: querying, after the secondtime period, passive tags having nodes with a second logic value, butnot passive tags having nodes with a first logic value.
 16. The methodof claim 13 wherein the subset includes a single passive tag.
 17. Themethod of claim 13 wherein a web includes the plurality of passive tags.18. The method of claim 13 wherein a web includes the plurality of strapassemblies.
 19. The method of claim 13 further comprising testing thesubset for operational performance.
 20. The method of claim 19 furthercomprising advancing a web and illuminating another subset.
 21. Themethod of claim 13 wherein each of the plurality of passive tagsincludes at least one persistent node unaffected by illumination tolight.
 22. A method for selecting a subset of radio frequencyidentification (RFID) tags comprising: broadcasting an interrogatingsignal to set a persistent node for each of the plurality of passivetags to a first logic value, the node capable of maintaining the firstlogic value for a first time period in the absence of power;interrupting power to the passive tag for a second time period byceasing the broadcasting, the second time period being shorter than thefirst time period; and illuminating a subset of the plurality of tagswith an illuminating source during at least a portion of the second timeperiod, persistent nodes of the subset degenerating to a second logicvalue during the second time period, wherein the interrogating signal isa broad beam relative to the narrow beam of the illuminating source.